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          <h1 class="post-title" itemprop="name headline">探究java8流收集数据原理</h1>
        

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        <div id="vip-container"><p>本文揭示如何学习一门新技术，从示例入手，重点阐述Stream#collect方法的实现原理，为更好的使用java8中流来收集数据。</p>
<p>我们在前面的文章中反复使用的场景：获取菜单中所有菜品的名称，返回一个集合，其代码如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">test_toList</span><span class="params">(List&lt;Dish&gt; menu)</span> </span>&#123;</span><br><span class="line">    List&lt;String&gt; names = menu.stream().map(Dish::getName)</span><br><span class="line">                        .collect(Collectors.toList()); <span class="comment">// @1</span></span><br><span class="line"></span><br><span class="line">    <span class="keyword">for</span>(String s : names) &#123;</span><br><span class="line">        System.out.println(s);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>通过Stream.collect这个终端操作进行数据收集，至于如何收集，则由该方法的参数来决定(Collector)，即行为参数化。</p>
<p>代码@1：collect(Collectors.toList()) 的意思就是返回List，这里涉及到两个关键，一个是Stream#collect方法，另外一个就是其参数Collectors.toList()。</p>
<p>接下来我们将以上述两个突破点来揭开如何使用java8的流来收集数据。</p>
<h2 id="1、Stream-collect"><a href="#1、Stream-collect" class="headerlink" title="1、Stream#collect"></a>1、Stream#collect</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">ReferencePipeline#collect</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">final</span> &lt;R, A&gt; <span class="function">R <span class="title">collect</span><span class="params">(Collector&lt;? <span class="keyword">super</span> P_OUT, A, R&gt; collector)</span> </span>&#123;   <span class="comment">// @1</span></span><br><span class="line">    A container;</span><br><span class="line">    <span class="keyword">if</span> (isParallel()</span><br><span class="line">            &amp;&amp; (collector.characteristics().contains(Collector.Characteristics.CONCURRENT))</span><br><span class="line">            &amp;&amp; (!isOrdered() || collector.characteristics().contains(Collector.Characteristics.UNORDERED))) &#123;   <span class="comment">// @2</span></span><br><span class="line">        container = collector.supplier().get();                                                                 <span class="comment">// @3</span></span><br><span class="line">        BiConsumer&lt;A, ? <span class="keyword">super</span> P_OUT&gt; accumulator = collector.accumulator();</span><br><span class="line">        forEach(u -&gt; accumulator.accept(container, u));</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">else</span> &#123;                                                                                                     <span class="comment">// @4</span></span><br><span class="line">        container = evaluate(ReduceOps.makeRef(collector));                                                                                              </span><br><span class="line">    &#125;</span><br><span class="line">    <span class="keyword">return</span> collector.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)                    <span class="comment">// @5</span></span><br><span class="line">           ? (R) container</span><br><span class="line">           : collector.finisher().apply(container);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>代码@1：函数声明，该方法返回的结果类型为R，传入的行为参数接口为Collector。<br>代码@2：判断是否符合并行化累积与规约的条件。</p>
<ul>
<li>是否是并行流，例如上例中创建流的代码为menu.stream()，该方法的流是非并行化流，如果要支持并行化执行，需要满足的第一个条件就是需要使用menu.parallelStream()方法返回的流。</li>
<li>Collector(收集器，行为化参数)中收集器行为集合中是否包含Characteristics.CONCURRENT(并行执行)，如果不包含该行为，则不支持并行执行。</li>
<li>原始流是否有顺序 或 者 收集器的行为集合中明确包含Characteristics.UNORDERED(不要求顺序性)。<br>上述三个条件必须同时满足，才能并行执行，否则串行执行。</li>
</ul>
<p>代码@3：并行执行收集动作。</p>
<p>代码@4：串行执行收集动作。</p>
<p>代码@5：如果收集器收集行为集合中包含Characteristics.IDENTITY_FINISH，则直接返回原始值，否则使用Collector.finishier()方式对计算的值进行函数式计算。</p>
<p>通过上面的代码，我们应该对Characteristics枚举类型中的3个值不难得出如下类型：</p>
<ul>
<li>CONCURRENT<br>收集器行为，表示收集其中的累积函数是否支持并行执行。</li>
<li>Characteristics.UNORDERED<br>收集器行为，表示整个收集期间，没有顺序要求。</li>
<li>Characteristics.IDENTITY_FINISH<br>收集器行为，表示可以忽略Collector.finsher()定义的最终转换函数，直接返回累积之后的结果即可。</li>
</ul>
<blockquote>
<p>疑问？代码@3,这段代码不是很好理解，该怎么继续往下深入呢？</p>
</blockquote>
<p>针对上面看不太懂的代码，我的处理办法是先转移思路，看一下Collector接口以及示例中Collectos.toList()返回的收集器是什么(重点关注返回的Collector中具体属性)。</p>
<span id="more"></span>

<h2 id="2、Collector接口"><a href="#2、Collector接口" class="headerlink" title="2、Collector接口"></a>2、Collector接口</h2><p><img src="https://img-blog.csdnimg.cn/20190604211337662.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ByZXN0aWdlZGluZw==,size_16,color_FFFFFF,t_70" alt="在这里插入图片描述"><br>收集器中的泛型参数说明如下：</p>
<ul>
<li>T ：累积器中一个操作数类型</li>
<li>A： 累积器中的初始值类型</li>
<li>R：返回值的类型，例如List&lt; R &gt;。</li>
</ul>
<p>其属性一览如下：</p>
<ul>
<li>Supplier&lt; A &gt; supplier()<br>该函数式接口，大家应该都非常熟悉了，其函数声明如下：() -&gt; T，通常用于构建对象，那这里是构建什么对象呢？这是下一个待解疑问。</li>
<li>BiConsumer&lt;A, T&gt; accumulator()<br>从名字命名来看，应该是返回累积器，（T,U）-&gt; void。通常用于输入两个参数，对其进行处理，但返回void类型。</li>
<li>BinaryOperator&lt; A &gt; combiner()<br>从名字命令来看，应该是组合器（请参考流计算函数reduce)。</li>
<li>Function&lt;A, R&gt; finisher()<br>最终函数，如果收集器行为包含IDENTITY_FINISH，则无需使用该函数对累积器产生的结果进行处理，否则使用该函数对累积器结果进行最后的处理。</li>
<li>Set&lt; Characteristics &gt; characteristics()<br>累积器行为，在上文已做详细介绍。</li>
</ul>
<p>其supplier函数到底是干什么的呢？对上面的方法都是基于名字来推测的（当然JDK代码非常优雅，根据名字去猜测，准确度还是很高的），但如何确认呢？这个时候我们还是结合Collectos.toList()方法返回的Collector来做进一步推断。</p>
<h2 id="3、Collectors-toList"><a href="#3、Collectors-toList" class="headerlink" title="3、Collectors.toList()"></a>3、Collectors.toList()</h2><p>代码如下：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">static</span> &lt;T&gt; Collector&lt;T, ?, List&lt;T&gt;&gt; toList() &#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">new</span> CollectorImpl&lt;&gt;((Supplier&lt;List&lt;T&gt;&gt;) ArrayList::<span class="keyword">new</span>, List::add,</span><br><span class="line">                               (left, right) -&gt; &#123; left.addAll(right); <span class="keyword">return</span> left; &#125;,</span><br><span class="line">                               CH_ID);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<ul>
<li>Collector的第一个参数Supplier，在这里为ArrayList::new，即调用supper.get()方法将返回一个List。</li>
<li>Collector的第二个参数accumulator:累积器，这里是List:add方法。</li>
<li>Collector的第三个参数combiner:组合器，这里就是(left, right) -&gt; {left.addAll(right);return left;}</li>
<li>Collector的第四个函数characteristics：收集器的行为，这里为CH_IL，其选项为：IDENTITY_FINISH。</li>
</ul>
<p>有了上面这些知识，我们再来看如下这段代码：<br><img src="https://img-blog.csdnimg.cn/2019060421180264.png" alt="在这里插入图片描述"><br>1、A container：累积器的初始值，如果使用Collectors.toList()，则这里会返回List<T>的对象。<br>2、获取collector中定义的累积器。<br>3、遍历流，执行累积器动作，其中形式参数u，代表流中的一个个元素。</p>
<p>至于forEach方法，底层流的具体实现，本文就不再往深探究。</p>
<h2 id="4、总结"><a href="#4、总结" class="headerlink" title="4、总结"></a>4、总结</h2><p>java8 使用流来收集数据的基本用法：</p>
<ul>
<li>使用流stream的collect对象进行数据收集，其参数为Collector函数是编程接口，具体的收集逻辑由该接口来指定。</li>
<li>流的收集其具备基本的属性即作用：<ul>
<li>Supplier&lt; A &gt; supplier()<br>通过该函数式编程接口，返回累积器的初始值。</li>
<li>BiConsumer&lt;A, T&gt; accumulator<br>累积器函数。</li>
<li>BinaryOperator&lt; A &gt; combiner<br>组合器，可以参考函数式编程接口的reduce方法。</li>
<li>Set&lt; Characteristics &gt; characteristics<br>收集器行为。</li>
</ul>
</li>
<li>java8中的Collectors提供了很多默认的收集器，例如Collectors.toList()方法，下一节我们会根据该类，详细介绍在java8中默认提供的收集器，指导我们如何使用java8中的流来收集数据。</li>
</ul>
<p>下一节，将以Collectos类为入口，详细介绍java8中默认提供的收集器，已经如何使用。</p>
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